The present invention relates to a vibration testing machine for subjecting an object to vibration of a prescribed amplitude and frequency. Although initially intended for cross axis testing of crash sensors, used as part of an automobile air bag restraint system, the machine has more general capabilities.
It is common practice to subject products that are being developed for the military, automotive or other markets, to vibration tests to determine the weak points in the product design. In some cases these vibration tests are meant to subject the product to a vibration stress that would be equivalent to what the device would experience during its useful life. If the product fails the test it must be redesigned until the tests are passed before the product is released for production.
It is now becoming increasingly common to vibration test production parts on a sampling basis to assure that they continue to pass the required specification. Some manufacturers are extending this process to where all production parts are subjected to a screening vibration test. In other cases, a sample off the production line is given a level of vibration which is intended to cause the product to fail.
It has been found that if all of the production parts are given a screening vibration test during the manufacturing process, that failures in the field are reduced along with the cost of returned product and warranty repairs. Failures that are found during this screening process also result in improvements in the production processes and, therefore, in higher quality products.
It has also been found in the vibration to failure tests, that a measure of the stability of the production process results. When the products fail in a shorter time period in these tests, the manufacturer knows that something has changed in the materials or production processes. A study of the failed parts points to where the change took place and thus leads to improvement in the process and to higher and more consistent quality parts.
Normally these vibration tests are conducted using standard voice coil vibrators such as manufactured by MB Dynamics, Ling Electronics, Ling Dynamic Systems, Unholz Dickie and others. These vibrators are usually large heavy machines requiring a significant installation space and are usually located in a testing laboratory. For vibrators to be used on a production line, on the other hand, it is desireable that they be small, light and powerful.
There are other situations where a small, light, but powerful vibrator is required. For example, a device was needed to subject an automotive crash sensor to cross axis vibrations while the sensor is simultaneously being shock tested along its longitudinal axis.
Components used on automobiles are subjected to various environmental tests to demonstrate performance under all automotive environments. A radio, for example, must function just as well on a hot day in Arizona as it does on a cold day in Alaska and the steering system must work when the car is traveling on a bumpy road as well as on a smooth one. In fact, every product on an automobile must be tested under each environment that it is likely to encounter during the normal lifetime of the vehicle. The one exception to this rule has been automobile crash sensors which are used with airbags.
During the precise moment that a crash sensor must perform its function and sense that an automobile is crashing, it is simultaneously being subjected to severe vibrations in all directions, yet never during the development and production of crash sensors are they subjected to these environments because, heretofore, no equipment has been available to conduct these tests. This is particularly significant since it is now well known that many crash sensor designs are significantly affected by vibrations that take place during crashes which can cause a significant delay in when the sensor triggers. Recent tests have shown that the vibration environment during a crash can even prevent some sensors from functioning. This resulting late deployment or non-deployment can cause serious injuries to vehicle occupants.
To solve this problem, a small, light, powerful vibrator could be mounted on the arm of a shock machine such as described in co-pending Castelli patent application Ser. No. 7/531,906 filed Jun. 1, 1990. Such a combination permits the simultaneous vibration and shock testing of a device such as a crash sensor.
The development of such a vibrator permits, in a similar manner, vibrators to be mounted onto other vibrators to give two and three axis vibration capability especially if the principles of rotary motion are used as disclosed herein.
There is a particular need for devices which will simultaneously vibrate a product in two or three axes for production screening as described above. If this screening requires the vibration of the product about three axes, it would cut the testing time down by at a factor of 3 if this can be done at the same time.
Conventional two or three axis vibrators connect each of the vibrators to the mounting plate through hydrostatic bearings thus isolating each axis from the motion of the other. These bearings make use of a pressurized fluid film between each vibrator and the mounting plate which requires provision for containing the fluid which is constantly leaking out of the hydrostatic bearing assemblies and a pump for creating the fluid pressure. This adds excessive weight and complexity and expense to the system.